[TRLog] MFJ Voice Keyer Interface

Tom Delker k1ky@bellsouth.net
Mon, 17 Aug 1998 11:15:30 -0500

I have sucessfully interfaced the MFJ 432 to my lapop computers for use
with TR-Log!

There were two problems with this device and the solutions are:

1. Severe hum and feedback when powering the unit from radio power.
    Solution:  Jumper (short out) L6 - a ground return choke in series
              with the mic and ptt ground leads.  This appeared
              to be an attempt to keep RF out, but what good is 
              it if you can't even use the unit before the TX is

2. Only DVK channel 1 would key when interfaced directly to the computer
   parallel port.
    Solution: Put 1N914 diodes in series with keying lines to DVK 
             channels 1,2 & 4 (not channel 3) from the computer
             parallel port.  It is possible that the diode on 
             channel 4 may not be needed either, but 1 & 2 are
             a must!
    Reasoning: The internal inputs of these keying lines have diodes
          from Channel 2 to 1, Channel 3 to 2.  This causes
           a path back to ground through the parallel interface
           causing the fired input to be held 
          "low" through the internal diodes and prevent the firing 
          channels from reaching a high enough logic level to work.

I have this setup on two laptops and keyers.  They perfomed perfectly
during the NAQP SSB contest this weekend.

Thomas M. Delker - K1KY CFO#395 - Smyrna / Gallatin / Short Mtn. TN
A.R.R.L. District Emergency Coordinator - Tennessee District #4
C.A.T.S. (W4CAT) - 147.30/145.23/145.49/145.17/223.98/444.45/444.65
Stones River A.R.C. (K4FUN) - http://www.members.home.com/srarc
Tennessee Contest Group (K4TCG) - http://www.k4ro.net/tcg.html
Fistfighters United Nashville (NT4L) 146.955 (114.8)

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